There are numerous known applications of polymer-coated fibre-based packaging materials, in which one or both sides of the fibre base are provided with one or more successive coating layers and which are intended for various packaging purposes. The coating can optionally achieve a moisture, oxygen and/or light barrier, which is intended to improve the storage life of the packaged product and thus to extend its effective storage and/or “best before date”. The coating also provides a heat-sealable material, enabling a tight seal of the product package. Food packages constitute one principal field of application for such packaging materials, which are used for instance for cartons, casings and boxes formed of coated packaging board and pouch packages formed of coated paper.
Heat-sterilised packages form a notable group of sealed food packages, the packaging material of these being required to withstand sterilising treatment conditions. Sterilisation may be performed by irradiation or by hot pressurised steam in an autoclave Closed metal cans have typically been treated in autoclaves, however, nowadays also fibre-based packaging materials are available for autoclave applications. WO 02/28637 thus describes a packaging material for autoclave packages, in which the board substrate is equipped with an oxygen barrier layer, polymer heat-sealing layers and binder layers required between these. In this publication, the oxygen barrier layer is preferably formed of an aluminium foil, even though EVOH is also mentioned as an optional material. The proposed material for the binder and heat-sealing layers is polypropene, which resists, without melting, the temperature above 130° C. prevailing in an autoclave.
In tests of heat-sterilised fibre-based food packages, the applicant has found the problem of bleached packaging board coated merely with polymer oxygen-barrier and heat-sealing layers tending to yellow due to heating. This is especially true for packages treated in autoclaves, which are required to resist the high autoclave temperature and pressure over the required treatment period, about 30 minutes in general. In other words, a package made of polymer-coated board gets another appearance during autoclave treatment, and this is a drawback in the market, especially in demanding applications.